1. Field of the Invention
The present invention relates to a photopolymerization type photosensitive flourescent paste composition and a method for forming a fluorescent film by using the same, and more particularly, to a photopolymerization type photosensitive phosphor paste composition useful for formation of a fluorescent film in a plasma display panel which is one of flat panel display, and a method for forming a fluorescent film in a plasma display panel by using the same.
2. Background of the Related Art
Being one of flat panel displays utilizing a phenomenon in which a vacuum UV ray(a wavelength approx. 147 nm) emitted from plasma formed at discharge of an inert gas is turned into red, green and blue lights within a visible light range when the UV ray makes collision onto red, green and blue flourescent films, the plasma display panel(called as xe2x80x98PDPxe2x80x99 hereafter) is paid attention as one of the next generation displays, such as HDTV, in view that the PDP can make a full color display, has a fast response and a wide angle of view, and is easy to realize a large sized display over 40xe2x80x3. The fluorescent film in the PDP has thin films of fluorescent materials which can emit red, green, and blue colors respectively coated inside of space surrounded by a barrier with a height 150 xcexcm at every 200 nmxcx9c300 nm formed on a lower glass substrate of the PDP, makes the full color of the PDP available, and is an important component which fixes a quality of the PDP as the flourescent film controls a luminance of the panel. The PDP fluorescent film is to be formed by screen printing and photolithgraphy. However, the screen printing has a problem in that a small contact area between a screen mask and the glass substrate makes it difficult that a uniform thickness is obtained on the barrier sides and the glass substrate unless a special care is taken for controlling a fluidity of the flourescent paste. Particularly, in a case of an HDTV PDP with a size larger than 40xe2x80x3, it is very difficult to apply a uniform coat of flourescent paste between barrier precisely throughout the glass substrate by the screen printing. Eventually, a photolithography using a liquid photosensitive flourescent paste is suggested as a screen printing method suitable for a large sized HDTV PDP.
As a material for forming the PDP flourescent film by the photolithography, a PVA-ADC type photosensitive flourescent paste is suggested in U.S. Pat. No. 5,086,297 in 1989, which is prepared by dissolving ammonium dichromate(call as xe2x80x98ADCxe2x80x99 hereafter) in a water solution of polyvinyl alcohol(call as xe2x80x98PVAxe2x80x99 hereafter) used in formation of a flourescent film in a color TV CRT, and dispersing flourescent material therein. However, since a structure and materials of the PDP are different from the TV CRT, the dichromate residue degrades the PDP flourescent material and drops a luminance during operation of the PDP. And, U.S. Pat. No. 5,136,207 in 1990 and U.S. Pat. No. 5,601,468 in 1996 suggest use of a PVA-DAST type photosensitive flourescent paste in formation of the PDP flourescent film, which is prepared by dissolving 4,4xe2x80x2-diazidostilbene-2,2xe2x80x2-disulfonic acid sodium salts, one of water soluble photosensitive agents, in PVA water solution and dispersing flourescent material. However, as developments of the PDP advance, there have been problems in that formation of a uniform flourescent film is difficult because, not only formation of a reflection type flourescent film on a PDP panel with a non-flat structure, of a photo cross-linking photosensitive paste, such as PVA-ADC and PVA-DAST is difficult, but also the photo cross-linking photosensitive paste has a weak adhesive force. And, a photopolymerization type photosensitive flourescent paste using acrylate group polymer having carboxyl group as a binder is known, the paste has disadvantages in that a high baking temperature is required after formation of the flourescent film, residual inorganic alkali metals gives an influence to the flourescent material after the development, and the alkali development solution causes environmental pollution.
The foregoing related art photo cross-linking type photosensitive flourescent paste will be explained in more detail. FIG. 1 illustrates a photoreaction mechanism of the related art photo cross-linking type photosensitive flourescent paste of PVA-ADC, schematically. FIG. 2 illustrates a photoreaction mechanism of the related art photo cross-linking type photosensitive flourescent paste of PVA-DAST, schematically.
The related art photo cross-linking type photosensitive flourescent paste of photosensitive polymer composition, such as PVA-ADC and PVA-DAST, is prepared by dissolving PVA-ADC or PVA-DAST which induces a photo cross-linking reaction in water, a solvent, arid dispersing one of red, green, and blue flourescent material powder therein, wherein the flourescent material powder is included in the photosensitive flourescent paste by 20xcx9c30 wt %, the water, a solvent. by 20xcx9c30 wt %, with balance of PVA-ADC or PVA-DAST. Upon coating and drying the photosensitive flourescent paste on the glass substrate having the barriers formed thereon, most of the water, a solvent, is vaporized, and the photosensitive polymer composition of PVA-ADC or PVA-DAST encapsulate the flourescent powder, to form micron grains of diameters ranging 3 xcexcmxcx9c6 xcexcm, which are stacked to form a composite membrane of flourescent material photosensitive polymer of a thickness ranging approx. 20xcx9c30xcexcm. Upon directing an UV ray to the composite membrane of flourescent material/photosensitive polymer with a mask placed thereon, an exposed portion is altered into megapolymer groups of three dimensional network structure by a photo cross-linking reaction of a mechanism as shown in FIGS. 2 and 3, and becomes insoluble in water, to remain during the development, while portions not exposed are removed, permitting to form a desired flourescent material pattern. In this instance, as shown in FIG. 1, in the PVA-ADC group photoreaction mechanism, a Cr(VI) ion included in the ADC is altered into Cr(III) ion by the UV ray to form a complex with an xe2x80x94OH in PVA, resulting to form a megapolymer group which is not soluble in water, which is a solvent. And, as shown in FIG. 2, in the PVA-DAST group photoreaction mechanism, DAST which is a water soluble photosensitive agent is decomposed by an UV ray, to form nitrene groups, which makes a hydrogen abstraction reaction with PVA, a polymer, to form a photo cross-linked polymer, to become insoluble in water, a development liquid. Thought he photo cross-linking photosensitive flourescent paste of a photosensitive polymer composition, such as PVA-ADC or PVA-DAST is applicable in formation of flourescent film in a color TV CRT with a thickness ranging 10 xcexcm, the paste is not applicable in formation of the PDP flourescent film having a thickness ranging 20xcx9c30 xcexcm before exposure due to a poor adhesive force coming from a difference of cross-linking densities. This is because the cross-linking density is varied with a number of chrome ions in DAST or ADC, a photosensitive agent, in the photo cross-linking photosensitive flourescent paste, which makes a direct bonding with PVA, a polymer, and such reaction can be occurred only when an UV ray is directed onto photosensitive agents, such as DAST and Cr(VI), directly.
Therefore, as a photo energy of an UV ray becomes the less due to absorption and reflection by the flourescent material grains as the film goes the thicker, the cross-linking may not be occurred or inadequate due to lack of the photo energy of the UV ray at a portion the film is required to make bonding to the substrate. Therefore, the contact portion, which is the deepest portion of the film, between the glass substrate and the flourescent film remained in a state not being cross-linked is bulged or dissolved by a development liquid during a development, leading to lost an exposed flourescent portion which should be remained.
Accordingly, the present invention is directed to a photopolymerization type photosensitive phosphor paste composition, and a method for forming a fluorescent film in a plasma display panel by using the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a photopolymerization type photosensitive phosphor paste composition, and a method for forming a fluorescent film in a plasma display panel by using the same, which has a low baking temperature after formation of flourescent film and is easy to form a flourescent film of a non-flat type for a PDP of AC or DC type.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the photopolymerization type photosensitive flourescent paste composition consists of a binder polymer which can be developed by pure water, a multifunctional monomer or oligomer, a photosensitizer which can make a photoreaction by an UV ray, a flourescent material, a solvent for dissolving the binder polymer, additives, and pure water.
In other aspect of the present invention, there is provided a method for forming a flourescent film in a PDP by using a photopolymerization type photosensitive flourescent paste composition, including the steps of (1) preparing red, green and blue photopolymerization type photosensitive flourescent paste composition consisting of water soluble binder polymer 1xcx9c15 wt %, multifunctional monomer or oligomer 3xcx9c7 wt %, photoinitiator 1xcx9c3 wt %, a solvent which can dissolve the binder polymer 20xcx9c35 wt %, additive 0.1xcx9c1 wt %, pure water, and flourescent material 25xcx9c35 wt % respectively, (2) coating one of the prepared red, green and blue photopolymerization type photosensitive flourescent paste compositions on a glass substrate having barriers formed thereon, (3) drying the coated composition, (4) exposing the dried composition and developing with pure water to form a first flourescent film, (5) processing the steps (2), (3) and (4) on one of the rest two photopolymerization type photosensitive flourescent paste compositions, to form a second flourescent film, (6) processing the steps (2), (3) and (4) on a finally remained Photopolymerization type photosensitive flourescent paste composition, to form a third flourescent film, and (7) baking the glass substrate having the first, second and third flourescent films formed thereon.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.